One method to control valve timing of an internal combustion having a variable event valvetrain is described in U.S. Pat. No. 6,681,741. This patent describes several valve timing methods that may be used during starting of an engine having a variable event valvetrain. In particular, the different methods attempt to improve cylinder turbulence and reduce fuel adherence to the intake port. The intake valves are initially set to open at a location that is before top-dead-center of the intake stroke of the cylinder in which the valve operates. After the engine speed reaches a predetermined level the intake valve opening time is switched to position that is after top-dead-center of the intake stroke of the cylinder. The intake valve opening remains at this location until a change in an operating condition causes the intake valve opening time to return to a position before top-dead-center of the intake stroke.
The above-mentioned method can also have several disadvantages. Specifically, the cylinder air-fuel mixture may not mix as well as desired when the engine is being cranked or when it is accelerating from crank speed up to the time it reaches a certain operating speed (run-up). Further, engine emissions may be higher than desired because the cylinder charge motion and air-fuel mixing may not be sufficient at low engine speed to improve combustion. In other words, portions of the cylinder air-fuel mixture may combust less completely than other portions of the cylinder air-fuel mixture due to lack of homogeneity of the air-fuel mixture. Further still, during engine crank and run-up, the limited charge motion may influence the lean combustion limit of the engine and thus may make it more difficult to start the engine with a lean air-fuel mixture. The ability to start the engine with a lean air-fuel mixture can be useful for lowering starting emissions since less hydrocarbons may be exhausted after combusting a lean air-fuel mixture, at least during some conditions.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method of controlling intake valve timing that offers substantial improvements.